How Can I Subdivide Only Part of a Grease Pencil Stroke (Ver. 2.92.0)?
Select only two points where you want to subdivide, and select Stroke &gt; Subdivide.

Or just right click on the context menu and select subdivide.

Note that you can modify the settings for the tools and the post processing for the draw tools to get better detail and optimize how control points are distributed on the drawn lines.

For more information read the manual https://docs.blender.org/manual/en/latest/grease_pencil/modes/draw/tools/draw.html#tool-grease-pencil-draw-draw